Conventionally, an air flow measurement device, which measures an air flow rate by utilizing heat transfer with air, is generally known. The air flow measurement device is arranged, for example, in an intake passage communicating with an internal combustion engine, and is used for measuring a flow rate of air flowing into the internal combustion engine.
For example, as shown in FIG. 5, an air flow measurement device 100 is arranged in an intake passage 101, and introduces therein a part of intake air to generate an electrical signal in accordance with a flow rate of the introduced air. The air flow measurement device 100 includes a bypass passage 103 through which a part of the introduced air flows, and the bypass passage 103 accommodates therein a sensor chip 102 which generates an electrical signal by heat transfer with the air. The sensor chip 102 of the air flow measurement device 100 is not arranged directly in the intake passage 101 through which a main flow of intake air passes, but is arranged in the bypass passage 103 not to be affected by turbulence of the main flow of intake air flowing in the intake passage 101. Thus, the sensor chip 102 can output a measurement value having little variation.
Moreover, because the sensor chip 102 is arranged in the bypass passage 103, a measurement value of the sensor chip 102 is restricted from decreasing from a true value, which is caused by pulsation of the main flow of intake air. The measurement value of the sensor chip 102 can be increased in accordance with increase of a flow length ratio L2/L1. Here, L1 is a flow length of air, which flows straight in the intake passage 101 without introduced into the air flow measurement device 100, from a position corresponding to an inlet of the air flow measurement device 100 to a position corresponding to the outlet of the air flow measurement device 100 in a flow direction of the main flow of the intake air, and L2 is a flow length of air flowing totally in the bypass passage 103. That is, the air flow measurement device 100 has a correction function, such that the decrease of the measurement value due to the pulsation can be prevented by setting the ratio L2/L1 to be a desired value.
In recent years, an amplitude of the pulsation of intake air tends to increase with the spread of exhaust gas recirculation (EGR), and therefore, a counter flow is generated periodically. To that end, technologies are proposed, in which influence of the pulsation on the measurement value (i.e., decrease of the measurement value) is limited by reducing an inflow of the counter flow into the bypass passage 103.
For example, in an air flow measurement device described in Patent Document 1 (JP 2007-309909A corresponding to US 2007/0056363A), a side wall of an outlet of a bypass passage has a slit, and a part of a counter flow is let out through the slit to reduce a flow rate of the counter flow which reaches a sensor chip.
In an air flow measurement device described in Patent Document 2 (JP 07-113672A), a gate which guides a counter flow is provided separately from an outlet of a bypass passage. The counter flow introduced into the gate is blown out to a lateral side of another counter flow, which is introduced into the bypass passage through the outlet of the bypass passage. Accordingly, the counter flow having entered into the bypass passage through the outlet is directed toward another passage which does not have a sensor chip. Thus, a flow rate of a counter flow, which reaches a sensor chip, is reduced.
In an air flow measurement device described in Patent Document 3 (JP 07-209051A), similarly to the air flow measurement device described in Patent Document 2, a counter flow, which flows into a bypass passage through an outlet of the bypass passage, is directed toward another passage which does not have a sensor chip. Additionally, the flow passage, to which the counter flow introduced from the outlet of the bypass passage is directed, also has another sensor chip, and is capable of measuring a flow rate of the counter flow. However, in any one of the above-described air flow measurement devices, the counter flow in the bypass passage cannot return to the intake passage if a flow direction of the counter flow introduced into the bypass passage is not changed. Thus, if the amplitude of the pulsation of intake air more increases, it may become difficult to effectively limit the influence of the counter flow.